The present invention relates to a tubular belt conveyor and more particularly to a tubular belt conveyor which comprises a belt guiding device for preventing a belt from loosening or expanding so as to assure smooth running.
As shown in FIG. 10, there is a conventional tubular belt conveyor in which an endless band-shaped conveyor belt 1 is rolled up into a tubular shape, flattened portions at the front and rear ends are wound around a front and a rear end roller to convey material which is thrown onto the front end of a forward belt 1a from a hopper 4 and is discharged onto a receiver 5 at the rear end.
The portion between a flat and a tubular portion or between a tubular and a flat portion is called "a through converting portion", and the distance therebetween is called "a trough converting distance".
In the conventional tubular belt conveyor, nylon or steel is employed as core material for the endless conveyor belt 1. As shown by a solid line in FIG. 11, the core material made of nylon is liable to lengthen and the trough converting distance is small, while what is made of steel is difficult to lengthen and the trough converting distance is large, as shown in a dotted line.
FIG. 11 shows a triangle representing the relationship between the elongation and the trough converting distance for an endless conveyor belt made of different material. The center line and both the side ends of the conveyor belt 1 are corresponding to the base and the hypotenuse of the triangle respectively. That is to say, the hypotenuse is longer than the base, which means that the side ends of the belt 1 are stretched longer than the center line.
Generally, the trough converting distance of the endless conveyor belt 1 is determined to keep the elongation less than 1%. The elongation less than 1% is within elasticity, and when the belt is formed into a tubular shape, the tubular belt becomes to have the same elongation over the whole width. In the endless steel-core conveyor belt 1, the trough converting distance lengthens so that the difference between the hypotenuse and the base becomes smaller.
As shown in FIG. 12, if the trough converting distance lengthens, the trough converting portion at the beginning part of the return belt 1b loosens by its weight, and the loosened portion increases because of the frictional resistance to which it is subject when it runs through a belt shape maintaining frame 6. Therefore, when the endless conveyor belt 1 is driven, said loosened trough converting portion moves up and down to cause surging action, so that the conveyor belt 1 travels intermittently. In other words, the running speed of the conveyor belt is not constant and the intermittent shock occurs in the belt, so that it is subject to large tension. Therefore, the front and rear end rollers around which the conveyor belt 1 is wound undergo strong force.
Also, if the conveyed material supplied from the hopper 4 is loaded ununiformly on the forward belt 1 at the trough converting portion of the beginning part, the forward belt 1a totally twists, or the side end of the belt expands over the usual tubular form because of the weight of the conveyed material, so that the elongation of the side ends of the belt increases, and the load of the belt and the weight of the conveyed material make the belt loosened, which results in the problems as mentioned above.